1. Field of the Invention
The present invention relates to a power supply apparatus and it particularly relates to a switching regulator.
2. Description of the Related Art
Inside a variety of electronic apparatuses, DC-DC converters, such as step-down type switching regulators and the like, are widely used to supply proper voltage to electronic circuits used therein. For example, when a voltage of 20 V only is available in an electronic apparatus and an electronic circuit therein is in need of 5 V voltage, the electronic circuit is driven by stepping down the 20 V to 5 V with a step-down type switching regulator.
The switching regulator like this has a control circuit for controlling the ON and OFF operations of the switching transistor. However, for a stable operation of the switching regulator, it is a matter of course that the supply voltage for the operation of the control circuit needs to be supplied stably. Now, if the supply voltage necessary for the operation of the control circuit is 5 V, then the 20 V prepared by the electronic apparatus cannot be used directly, so that a conceivable method is to generate the 5 V by a linear regulator. However, there has been a problem that the generally poor efficiency of a linear regulator leads to a worsened efficiency of the switching regulator as a whole.
To solve problems like this, a technology for using the voltage stepped down by a switching regulator as the supply voltage to drive the control circuit has been proposed (Patent Document 1). According to this technology, a voltage capable of driving the control circuit is used as a threshold value, and switching is made such that when the output voltage of the switching regulator is higher than the threshold voltage, the control circuit is driven by the output voltage, and when the output voltage of the switching regulator is lower than the threshold voltage, the control circuit is driven by a linear regulator. According to this technology, the power conversion efficiency of the switching regulator is higher than that of the linear regulator, so that a higher efficiency can be realized during the period when the supply voltage of the control circuit is supplied from the output voltage of the switching regulator.
Patent Document 1]
U.S. Pat. No. 5,528,132 (specification)
Nevertheless, the technology as described in the above-mentioned document has had a problem that when there is a variation of the output voltage of the switching regulator in the neighborhood of the threshold voltage, it is not stably determined which of the voltage of the linear regulator or the output voltage the supply voltage of the control circuit is supplied from. There is another problem that there is overshoot or undershoot of output voltage at the starting of the switching regulator, which causes erroneous operation of the circuit when the output voltage above a certain level of voltage is used as the supply voltage of the control circuit.